Chromium based catalysts are used in the commercial polymerization of small alpha-olefins such as ethylene and propylene. One such catalyst is prepared by depositing chromocene (bis(cyclopentadienyl) chromium (II)) on an inorganic metal oxide support, as disclosed in British Patent No. 1,253,063 to Karapinka. U.S. Pat. No. 4,015,059, issued Mar. 29, 1977 to Karol, describes the use of bis(indenyl)- and bis(flourenyl)-chromium (II) compounds supported on activated inorganic oxide supports as catalysts for the polymerization of ethylene.
Recently, new synthetic methods have been described for preparing Cr.sup.+3 organometallic compounds. Theopold, J. Am. Chem. Soc. (1988), 110, 5902 entitled "Cationic Chromium (III) Alkyls as Olefin Polymerization Catalysts", Theopold, Acc. Chem. Res. (1990), 23, 263 entitled "Organochromium (III) Chemistry: A Neglected Oxidation State" and Thomas et al., J. Amer. Chem. Soc., 113 (1991), p. 893 et seq. disclose that certain pentamethylcyclopentadienyl chromium (III) alkyls can be prepared, and that they can be used for making polyethylene homogeneously in CH.sub.2 Cl.sub.2. However, these homogeneous Cr (III) polymerization catalysts have several deficiencies. These include low polymer productivity, rapid deactivation, and the need to use polar, non-coordinating solvents. Additionally, since they are homogeneous catalysts, they are unsuitable for gas phase olefin polymerizations.
U.S. Pat. No. 4,530,914, issued Jul. 23,1985 to Ewen et al., discloses a catalyst system for the polymerization of alpha-olefins which comprises two or more metallocenes, each having different propagation and termination rate constants, and aluminoxane. The metallocenes are cyclopentadienyl derivatives of a transition metal of Group 4b, 5b, and 6b metals of the Periodic Table. They are described by the formulas (C.sub.5 R'.sub.m).sub.p R".sub.s (C.sub.5 R'.sub.m)MeQ.sub.3-p and R".sub.s (C.sub.5 R'.sub.m).sub.2 MeQ' where (C.sub.5 R'.sub.m) is a cyclopentadienyl or substituted cyclopentadienyl, each R' is hydrogen or a hydrocarbyl radical, R" is an alkylene radical, a dialkyl germanium or silicon or an alkyl phosphine or amine radical bridging two (C.sub.5 R'.sub.m) rings, Q is a hydrocarbon radical, Me is a Group 4b, 5b, or 6b metal, s is 0 or 1, p is 0, 1, or 2; when p=0, s=0; m is 4 when s is 1 and m is 5 when s is 0.
U.S. Pat. No. 4,939,217, issued Jul. 3, 1990 to Stricklen, also discloses a process for polymerizing olefins where the polymerization is conducted in the presence of hydrogen, and a catalyst system is used which contains aluminoxane and at least two metallocenes, each having different olefin polymerization termination rate constants. The metallocenes disclosed are similar to those described in aforementioned U.S. Pat. No. 4,530,914.
U.S. Pat. No. 4,975,403, issued Dec. 4, 1990 to Ewen, discloses a catalyst system for use in the polymerization of olefins. The catalyst system includes at least two different chiral, stereo-rigid metallocene catalysts of the formula R"(C.sub.5 (R').sub.4).sub.2 MeQ.sub.p (where Me is a Group 4b, 5b or 6b metal and (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl ring) and an aluminum compound.
Canadian Patent Application No. 2,000,567, published Apr. 13, 1990, discloses a process for producing polyethylene using a composite catalyst made up of a solid catalyst component typified by a selected chromium compound, a modified aluminum compound typified by a trialkylaluminum, and an alkylaluminum alkoxide compound. The chromium compound may be chromium oxide, and the modified aluminum compound may be the reaction product of an organoaluminum compound and water.
It has now been discovered that when cyclopentadienyl Group 6b metal hydrocarbyl compounds, in which the Group 6b metal is in an oxidation state of +3, are supported on inorganic metal oxide or inorganic metal phosphate supports, high productivity alpha-olefin polymerization catalysts are produced, and that the use of a co-catalyst improves the productivity of many of these compounds.